Liquid crystals are suitable as dielectrics in indicating devices, since their optical properties can be influenced by an electric potential. Suitable electro-optical devices are well-known to the person skilled in the art. Examples of such devices are cells having dynamic scattering, DAP cells (deformation of aligned phases), TN cells (twisted-nematic) and STN cells (super twisted-nematic) having a twisted nematic structure, guest/host cells, phase change cells having a cholesteric-nematic phase transition and SBE cells (super birefringence effect).
Further, electro-optical devices based on chiral tilted smectic liquid crystals are proposed in Appl. Phys. Lett. 36, 899 (1980) and in Recent Developments in Condensed Matter Physics 4, 309 (1981). In this case the ferroelectric properties of these materials are made use of. As the tilted smectic phases there are suitable, for example, smectic C, F, G, H, I and K phases. There are generally preferred smectic C phases which, in particular, permit high response speeds. The chiral tilted phases are usually denoted by S.sub.C.sup.*, S.sub.F.sup.* etc., with the asterisk indicating the chirality.
Ferroelectric liquid crystals should have a good stability towards chemical and thermal influences and towards electrical fields. Further, they should have a suitable mesophase over a broad temperature range, low viscosity and especially a sufficiently high spontaneous polarization.
Suitable mixtures of ferroelectric liquid crystals include one or more optically active doping substances and a liquid crystal material which may include one or more components and which as a rule should have a tilted smectic phase. The optically active doping substances need not themselves be smectic, but in the liquid crystal material they should produce a chiral tilted smectic phase and should induce a high spontaneous polarization. In order to achieve a high spontaneous polarization in the mixture and/or in order to hold the amount of optically active doping substances relatively low, doping substances which already, in small amounts, are capable of inducing a high spontaneous polarization are desired.
On the other hand, the pitch of the liquid crystal should be distinctly greater than the plate separation of the cell which is used and should typically amount to at least about 10 .mu.m in order to obtain bistable displays which switch well. Optically active doping substances for ferroelectric liquid crystals should therefore preferably induce in the mixture a chiral tilted smectic phase having not only a high spontaneous polarization but a comparatively small twisting (that is, with a large pitch).